In the introduction to the movie, it shows ash being sucked out of the cabin and into the desert, when you look at his back, the boomstick is gone. Yet, magically, when he is standing outside of the pit, it magically appears in his hands

The reason is explained in the Evil Dead companion. There was originally supposed to be a part around when the wise man gives him his chain saw where the wise man also gives him his shotgun, but it was cut due to time constraints.